Unblocking Temperature Component Research Articles

Effects of Pressure and Temperature Changes on Shock Remanence Acquisition for Single‐Domain Titanomagnetite‐Bearing Basalt

AbstractKnowledge of the shock remanent magnetization (SRM) property is crucial for interpreting the spatial change in a magnetic anomaly observed over an impact crater. This study conducted two series of impact‐induced SRM acquisition experiments by varying the applied field intensity (0–400 μT) and impact conditions. Systematic remanence measurements of cube‐shaped subsamples cut from shocked basalt containing single‐domain titanomagnetite were conducted to investigate the effects of changes in pressure and temperature on the SRM acquisition. The peak pressure and temperature distributions in the shocked samples were estimated using shock‐physics modeling. SRM intensity was proportional to the applied field intensity of up to 400 μT. SRM intensity data for peak pressure and temperature of up to 8.0 GPa and 530 K, respectively, clearly show that it increases with increasing pressure and decreases with increasing temperature. The SRM has unblocking temperature components up to a Curie temperature of 510 K, and it easily demagnetizes with alternating field demagnetization. The observed SRM properties can be explained by the pressure‐induced microcoercivity reduction and temperature‐induced modification of the blocking curve. Although the remanence acquisition efficiency of the SRM is significantly lower than that of the thermoremanent magnetization (TRM), the magnetic anomaly originating from the SRM distribution in a broader region may show a contribution comparable to that of the impact‐induced TRM distribution in a narrow region.

Palaeomagnetism of the mid-Cretaceous red beds from the Tethyan Himalaya: direction discrepancy and tectonic implications

Understanding the northern extension of Greater India is vital for modelling the India–Asia collision process and the formation of the Tibetan Plateau. We present new palaeomagnetic data from the mid-Cretaceous ( c. 106 Ma) Gyabula Formation red beds in the Tethyan Himalaya. Well-defined high laboratory unblocking temperature component magnetizations were isolated from 19 sites and pass the fold tests, indicating that they are pre-folding magnetizations. The tilt-corrected site-mean direction is D s = 222.9°, I s = +39.4° with α 95 = 4.2°. The site-mean inclination increases from 39.4° to 45.8° after anisotropy-based inclination shallowing correction. The declination and inclination differ considerably from those of neighbouring sections. This directional discrepancy of the red beds may be attributed to the fact that the sampled section (sites ZB1–11, 40–52) is overturned and to local vertical-axis rotation. The combination of our new and previously published palaeomagnetic data shows that the Tethyan Himalaya was located at 27.5° ± 2.4°S for the sampled area. Our new results, together with the reliable Cretaceous palaeomagnetic data obtained from the Tethyan Himalaya as well as coeval palaeolatitudes expected from the Indian craton, support a smaller Greater India and that the Tethyan Himalaya did not rift from the Indian craton during the mid-Cretaceous. Supplementary material: Detrital zircon U–Pb ages data, hysteresis parameters data, palaeomagnetic datasets for the Gyabula Formation, anisotropy of isothermal remanent magnetization data, available latest Jurassic–Early Cretaceous palaeomagnetic results from the Tethyan Himalaya and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6795683 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Rock magnetic study on the Yetti–Eglab Intrusions, Sahara: contribution to the West African Craton geology

AbstractNew palaeopoles have been calculated for the West African Craton (WAC) using palaeomagnetic data obtained on 178 cores of the Yetti–Eglab intrusions and stromatolite-bearing formation. One, two or three components of magnetization on 172 (14 sites) and 37 (6 sites) oriented specimens or cores were isolated mainly on doleritic dykes. High and low unblocking temperature components were evidenced in the dykes but also at four sites of the well-dated Hank stromatolite-bearing formation (875–890 Ma). Rock magnetic analyses show stable remanent magnetizations mainly carried by magnetite (or titanomagnetite) but may also reveal the presence of hematite, pyrrhotite and goethite. Regarding the recently constructed apparent polar wander path of the WAC until 500 Ma, these newly computed remanent components seem to be mainly older, some of which could be remagnetizations at different periods. However, according to geological dating and literature, the Yetti and Eglab terranes belonging to the WAC were amalgamated around 1.9 Ga and involved in the formation of the Columbia Supercontinent. The palaeopole computed for the stromatolite-bearing formation corresponds with the location of Rodinia Supercontinent at its early stages of amalgamation. The geodynamic evolution of the WAC with respect to both supercontinents suggests that the Eglab and Yetti could be separated by oceanic crust before 1.9 Ga. A volcanic arc developed during the subduction of this ocean followed by crustal thickening that generated an intrusive suite (Aftout granites) when it was at low northern latitudes. Baltica and Rio de la Plata cratons were close to the WAC after 1.7 Ga, following a nearly similar path between 1.7 and 0.9 Ga. In Africa, the collapse of the Birimian orogen was probably contemporaneous with the fragmentation of Columbia. At 0.9 Ga, Baltica was probably not attached to the WAC since the latter was not affected by the later Grenvillian orogen.

Extreme geomagnetic reversal frequency during the Middle Cambrian as revealed by the magnetostratigraphy of the Khorbusuonka section (northeastern Siberia)

We present new magnetostratigraphic results obtained for the Drumian stage (504.5–500.5 Ma; Epoch 3/Middle Cambrian) from the Khorbusuonka sedimentary section in northeastern Siberia. They complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time. Magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions. Thermal demagnetization makes it possible to distinguish two magnetization components. The low unblocking temperature (<350°C) component has a steep inclination and likely originates from remagnetization in a recent field. At higher temperatures, the magnetization isolated possesses the two polarities. Its direction is usually well determined; however, for a noticeable set of samples, a strong overlap between the demagnetization spectra of the two components prevents the determination of reliable directions, although their polarities are well established. The directions from 437 samples define a sequence of 78 magnetic polarity intervals, 22 of which are observed in a single sample. Biostratigraphic data available from the Khorbusuonka section indicate that the duration of the studied section is ∼3 Myr. A geomagnetic reversal frequency of 26 reversals per Myr is therefore estimated for the Drumian, reduced to 15 reversals per Myr if only the polarity intervals defined by at least two consecutive samples are retained. This is an extreme reversal rate, similar to that reported for the Late Ediacaran (late Precambrian), ∼50 Myr earlier, and proposed to be potentially linked to a late nucleation of the inner core. The reversal frequency appears to have drastically dropped for ∼3–4 Myr from a value probably >20 reversals per Myr during the Drumian to ∼1.5 reversals per Myr during the Furongian/Upper Cambrian. Such a sharp decrease is consistent with a transition at a ∼1-Myr timescale, probably caused by threshold effects in core processes, between two geodynamo modes, one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per Myr, and the other marked by hyperactivity of the reversing process, with reversal rates >15 reversals per Myr.

Large-scale rotations of the Chortis Block (Honduras) at the southern termination of the Laramide flat slab

Paleogeographic reconstructions place the Chortis block adjacent to southern Mexico in the Late Cretaceous and earlier time, forming the tectonically disturbed southwestern tip of the North American plate. This study assesses the relative motion between Chortis and North America during the Laramide orogeny, later development of the North America-Caribbean plate boundaries. We carried out paleomagnetic analysis of rotations and latitudinal displacement using a total of 90 sites from the Átima Limestone and the Valle de Ángeles Formation redbeds sampled at seven localities in Honduras. Two secondary but ancient magnetizations post-date Late Cretaceous folding at most localities. The oldest magnetization is a ChRM of high laboratory unblocking temperature in the redbeds (>650°C), and a moderate temperature in the limestones (500–600°C). This component is WNW directed at five localities, varying somewhat in declination, having a consistent moderate positive inclination (mean I=39.7°; α95=6.8°, n=53 sites). The age of the ChRM is bracketed between 85 and 63Ma, based on youngest zircon age and the age of a cross-cutting dike. An intermediate unblocking temperature component present in both units (Dec=146.2°, Inc=−28.5°; k=13.1, α95=6.8°, n=37 sites), is interpreted as an overprint. The ChRM indicates an average counterclockwise rotation of 55.4±5.7°, with respect to the expected direction. The younger overprint was acquired between ~50 and 15Ma; its mean is also discordant with the expected North America direction, implying some Neogene rotation of Chortis. Abundant late Albian-Turonian zircons in Valle de Ángeles redbeds explicitly require extending the Laramide arc along western Mexico into Chortis. A reconstruction of Chortis south of Mexico that takes into account 55 degrees of rotation shows a gap between both continental blocks at about 70Ma. We propose a model where this gap formed by back-arc extension creating the Late Cretaceous Chontal basin at the southern termination of the Laramide flat-slab.

Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin

The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~280m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150°C and ~300°C removes the secondary, moderately inclined (~48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~200m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF+UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles.

Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

AbstractWe studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

New paleomagnetic results from the Huaibei Group and Neoproterozoic mafic sills in the North China Craton and their paleogeographic implications

New paleomagnetic pole positions are obtained from the Neoproterozoic sills and carbonate rocks of the Huaibei Group in the northern Jiangsu and Anhui Provinces (Xuhuai region), North China. A total of 240 samples are used in statistics after stepwise thermal demagnetization and principal component analysis, including 85 samples from three ∼890Ma mafic sills, 27 samples from the ∼890-Ma-sill-baked rocks of the Wangshan Formation (Fm), 50 samples from unbaked rocks of the Wangshan Fm, 60 samples from unbaked rocks of the Jiayuan Fm and 18 samples from a ∼925Ma mafic sill. For most of these samples, after removing a low unblocking temperature component (LTC) of viscous magnetic remanence acquired in recent geomagnetic field, high unblocking temperature components (HTCs), likely carried by magnetite for the sill and unbaked carbonate samples and by partly oxidized magnetite for baked carbonate samples, were isolated. The HTCs of the ∼890Ma sills passed a reversal test, a fold test, and a baked contact test. It is interpreted as a primary remanence, defining a key pole (Q=7) at 52.6°N, 330.0°E (A95=5.3°) for the North China Craton (NCC). The HTCs of the unbaked carbonate rocks of the Wangshan Fm passed a fold test, the corresponding pole being at 26.1°N, 320.3°E (A95=5.2°). The sample-level mean direction of the ∼925Ma sill provides a virtual geomagnetic pole (VGP) position of 30.9°S, 136.2°E, dm/dp=4.0°/2.4°, being antipodal to the pole of the Wangshan Fm. We interpreted that the two rock units have the same age and their mean pole likely represents the primary remanences. A paleomagnetic pole position of 54.0°S, 107.3°E (A95=4.0°) is obtained from the Jiayuan Fm, by averaging the site-level VGPs. This pole is of dual polarity and consistent with those published poles from this formation and other three carbonate formations in the middle part of Huaibei Group, but differs from any younger poles obtained in the region. We also interpreted it as of a primary remanence. Our new data, together with the published poles, depicted a loop-like segment of apparent polar wander path of the NCC, with an apex at ∼925Ma. It is termed as the “Huaibei loop” herein. The comparison between the “Huaibei loop” and the “Sveconorweigian loop”, aided by the “right-way up” model of Laurentia–Baltica connection, indicates the paleogeographic proximities of the NCC to Laurentia, and probably to Siberia as well, in the supercontinent Rodinia.

The Alashan Terrane did not amalgamate with North China block by the Late Permian: Evidence from Carboniferous and Permian paleomagnetic results

Rock magnetic and paleomagnetic studies have been carried out on the early Carboniferous limestones and the Late Permian purple sandstones sampled in the eastern Alashan Terrane (ALT), northwest of China. Two components were isolated from the Early Carboniferous limestone by thermal progressive demagnetisation: a low unblocking temperature component (LTC) of recent origin; a pre-folding medium temperature component (MTC) (the paleomagnetic pole is λ=13.1°N, Ф=11.0°E, A95=7.0°) that is probably the result of the hydrothermal fluids from the Qilian Orgenic Belt acquired during the Late Carboniferous–Early Permian. Also, two components were separated from the Late Permian purple sandstone by thermal progressive demagnetisation: the LTC with the recent viscous remanent magnetisation, and the higher temperature component (HTC) revealed from three sections which has passed a regional fold test at the 95% probability level and reversal test, suggesting a primary characteristic magnetisation. The corresponding paleomagnetic pole is λ=27.2° N, Ф=18.8° E, A95=12.0°. The apparent polar wander path (including early Carboniferous, late Carboniferous-Early Permian, Late Permian and Early–middle Triassic poles) of the ALT is significantly different with those of the NCB. Comparison of the APWPs between the ALT and NCB shows a strong similarity. If the APWP of Hexi Corridor–Alashan rotated counterclockwise around an Euler pole at 44°N, 84°E by 32°, then the coeval APW path of the ALT overlaps to that of the NCB. This result indicates that the ALT migrated to the NCB after the Early–Middle Triassic along a tectonic boundary located between Helanshan Mountain and Zhuozishan Mountain, and finally amalgamated to the NCB before the Early Cretaceous.

Signals from the ancient geodynamo: A paleomagnetic field test on the Jack Hills metaconglomerate

The oldest history of Earth's magnetic field cannot be directly read from extant bulk rocks because of subsequent metamorphism at temperatures close to or exceeding the Curie temperature of common magnetic minerals. The Jack Hills metasediments of Western Australia, which have seen lower peak metamorphic temperatures, contain zircons as old as ∼4.4 billion-yr-old. To assess whether these sediments can retain an ancient signal of the geodynamo, we present a paleomagnetic conglomerate test on a cobble-bearing Jack Hills unit. Thermal demagnetization reveals a distinct magnetic component with high unblocking temperatures between ∼550 and 580°C that passes the conglomerate test, indicating magnetization prior to deposition of the conglomerate. This result, together with rock magnetic data, indicates that the high unblocking temperature component is carried by magnetite which records magnetization in an ambient field, and the simplest explanation is that a dynamo was present. Existing geochronological data imply that the clasts could contain mixtures of minerals extending to ages only slightly older than the maximum depositional age at 3.05 billion-yr-ago. However, the positive conglomerate test reported here indicates that the Jack Hills metasediments have the potential to record Paleorchean to Hadean magnetic fields, on a clast or sub-clast mineral scale.

Toward constraining the long-term reversing behavior of the geodynamo: A new “Maya” superchron ∼1 billion years ago from the magnetostratigraphy of the Kartochka Formation (southwestern Siberia)

We present new magnetostratigraphic data obtained from the Late Mesoproterozoic (∼1Ga) Kartochka Formation in the East Angara terrane of the Yenisey Ridge region (southwestern Siberian platform). A ∼200m-thick section encompassing the carbonate Kartochka Formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites. Paleomagnetic analyses revealed the existence of two magnetization components. A low unblocking temperature component, which likely has a viscous origin, was first isolated below 200°C. A characteristic component, carried either by magnetite or by a mixture of magnetite and hematite, was then isolated in the medium to high temperature range (up to 565–585°C or 680°C). The hematite-bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening. A positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition. This primary origin was further verified by comparison between the paleomagnetic poles derived for the Kartochka Formation and other late Mesoproterozoic Siberian sections. All data from the Kartochka Formation contain a single magnetic polarity assumed to be normal. This long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the Siberian Uchur Maya region. The new data fortify evidence of a normal polarity superchron spanning the ∼1000Ma old Mesoproterozoic–Neoproterozoic boundary, which we propose to call the Maya superchron. These data confirm the occurrence of sharp transitions between a frequently reversing regime and a non-reversing regime of the geodynamo. This behavior, also observed in other regions, may represent a consistent property of the Proterozoic geomagnetic field. Together with changes in the amount of time spent in the superchron regime, they may testify to a different field behavior during the late Precambrian than during the Phanerozoic. This difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core–mantle boundary during the Precambrian, possibly as a consequence of the inner core not yet being nucleated at this time.

Malley diabase dykes of the Slave craton, Canadian Shield: U–Pb age, paleomagnetism, and implications for continental reconstructions in the early Paleoproterozoic1Geological Survey of Canada Contribution 20110114.

The NE-trending Malley dyke swarm, dated herein at 2231 ± 2 Ma (U–Pb baddeleyite), extends from the central Slave craton to the vicinity of the Kilohigok basin, and may continue farther to the northeast as the geochemically similar Brichta dyke swarm, having been offset sinistrally along the prominent Bathurst fault. It carries a characteristic high unblocking temperature paleomagnetic component of single polarity directed up SE (mean direction: D = 138.3°, I = –53.8°), with corresponding paleopole at 50.8°S, 50.0°W. Lower unblocking temperature components, in some cases directed down SE, similar to ca 1.75 Ga post-Hudsonian overprints, are easily removed using combined alternating field (AF) thermal demagnetization, but difficult to remove using AF cleaning alone. The characteristic remanence has not been demonstrated primary, but is significantly older than 2.03 Ga, the age of Lac de Gras dykes, based on a baked contact test at a Lac de Gras – Malley dyke intersection. In addition, an E- to ESE-trending dyke carries a down WNW remanence, typical of 2.19 Ga Dogrib dykes near Yellowknife, suggesting that regional overprinting has not affected the study area since Dogrib emplacement, and that the Malley remanence was acquired at or shortly after Malley emplacement. Comparing Malley and Lac de Gras paleopoles with the 2.22–2.00 Ga Superior craton apparent polar wander path indicates that the two cratons were (i) not in their present relative orientation at 2.23 or 2.03 Ga, and (ii) likely not drifting in close proximity to one another as parts of a single (super)continent throughout the 2.23–2.03 Ga interval.

Palaeogeographic implications of differential inclination shallowing in permo-carboniferous sediments from the donets basin, Ukraine

We present new palaeomagnetic data from Upper Carboniferous and Lower Permian grey and red sediments from the Donets Basin, Ukraine, part of the Palaeozoic East European Platform. Detailed demagnetization of these units reveals two ancient components of magnetization: component “B”, which is carried by magnetite and pigmentary haematite, and a high unblocking temperature component “C”, present only in the red beds, carried by detrital haematite. The “B” and “C” components both pass fold tests indicating a primary or a near primary-origin for magnetizations. The “C” component, however, yields palaeolatiudes that are consistently lower (by up to ∼ 12° of latitude or ∼ 1330 km) than those derived from the “B” component, and we argue that this is due to significant inclination-shallowing of the “C”-component. A comparison with European reference palaeomagnetic data reveals that the reference data also span a large spread of palaeolatitudes for this time, and we argue that unrecognized shallowing may have crept into the reference data when based on sedimentary units. A more rigorous approach to selecting reference palaeomagnetic data may well be key to resolving palaeogeographic controversies at this time.

Paleomagnetism of the Precambrian Eastern Sayan rocks: Implications for the Ediacaran–Early Cambrian paleogeography of the Tuva-Mongolian composite terrane

The Tuva-Mongolian Precambrian composite terrane is located within the complex Central Asian fold belt, which separates the Siberian portion of the Eurasian continent from other incorporated continental blocks. This terrane is one of the key elements that must be accurately modeled to reconstruct the Neoproterozoic–Paleozoic tectonic history of Eurasia. The reconstruction of the terrane's paleoposition after breaking up of supercontinent Rodinia relatively to other continental blocks has fundamental importance, as it is one of the missing blocks in most present day reconstructions. We present a paleomagnetic study of rocks from several formations of the Eastern Sayan region south of Siberian platform (representative location λ = 52.0°N, ϕ = 100.5°E), within the Tuva-Mongolian composite terrane. Sections of siltstone, fine grained sandstone and associated sills were sampled at several localities in this complex region. These units have been dated as Precambrian to Early Cambrian in age. The resulting collection was taken from three formations, representing a total of 33 sites, collected from this previously unsampled and remote region. Generally 6 to 11 samples per site were collected. Stepwise thermal demagnetization was completed using between 10 and 18 heating steps to up to temperatures of 680 °C. Principal component analysis of the stepwise thermal demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may correspond to the present day Earth's magnetic field. The higher unblocking temperature magnetic component (B), was observed in the Dunzhugur Formation (B DF, N = 8 sites) and the Bokson Formation (B BF, N = 11 sites from 5 localities). The B component differs significantly from component A, and is recorded by sites of downward and upward directed magnetic inclinations in the Bokson Formation. Component B DF is most likely a result of the regional remagnetization of the sediments and sills during multiple tectonic events in the area. Component B BF yields a positive fold test and one site has reversed polarity direction. A virtual geomagnetic pole calculated from component B BF, after rotating along a small circle, is coincident with other Ediacaran to Early Cambrian aged poles reported from nearby Mongolia and Siberia. This observation supports the earlier interpretation of Kravchinsky et al. (2001) postulating an adjacent position of the Tuva-Mongolian composite terrane and the Siberian continent in Ediacaran–Early Cambrian times.

Evidence for a 3.45‐billion‐year‐old magnetic remanence: Hints of an ancient geodynamo from conglomerates of South Africa

Paleomagnetic and rock magnetic analyses of ∼3445‐million‐year‐old dacite conglomerate clasts and parent body rocks from the Barberton greenstone belt, South Africa, define two dominant components of magnetization. One component, unblocked at low temperature, is an overprint acquired ∼180 million years ago. The other component is unblocked at high temperatures and passes a conglomerate test, indicating that this component is older than the depositional age of the conglomerate (∼3416 Ma). The high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes, Archean overprinting, and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations. Alternatively, this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo, including magnetization by an external field related to solar wind interaction with the atmosphere. Such exotic mechanisms can be tested with the acquisition of paleointensity data. While more scattered than paleomagnetic data recording the more recent geomagnetic field, the high unblocking temperature component in the dacite parent body shows some consistency, and the simplest explanation of the data is that they reflect a geodynamo ∼3445 million years ago.

Paleomagnetic constraints on neotectonic deformation in the Kashi depression of the western Tarim Basin, NW China

A paleomagnetic study is reported of Eocene to Pliocene formations from the Kashi depression, which aims to constrain the pattern of neotectonic deformation within the western sector of the Tarim Basin in northwest China. With the exception of Pliocene specimens from one locality (East Kulukeqiati) which show large within site-mean variations in declination, most sites from five sampled formations yield well-grouped characteristic remanent magnetizations and positive fold tests and are of probable post-depositional detrital origin. First-order consistency of paleomagnetic results from a range of rock ages and localities demonstrates that only small inter-locational vertical-axis rotation has occurred here and indicates that the Kashi depression is decoupled from the remainder of Tarim to the east and has behaved as a quasi-rigid block which has rotated by 20–30° counterclockwise relative to Eurasia and North China since the late Pliocene. The crustal-scale Talas-Ferghana Fault cuts the Tian Shan and meets the Kashi depression in the region immediately to the northwest of the study region and we find no paleomagnetic evidence for differential rotations to suggest that this fault zone extends southwards across the Kashi depression to link with the North Pamir Thrust Fault (NPTF). Instead, we argue that the southern extension of this zone is a transform-orogen junction with southward motion of the eastern wall accommodated by southward thrusting at the margins of the south Tian Shan and the Tarim Basin. We propose that dextral transpression around the margins of the crustal block incorporating the Kashi depression was responsible for the contrasting amounts of thrusting on the NPTF in the southwest and the South Tian Shan Thrust Fault in the north. Extensive evidence for neotectonism in the bordering zones of this block, as well as some paleomagnetic evidence from low unblocking temperature components, indicates that the deformation produced by block rotation is ongoing.

Uplift and cooling magnetisation record in the Bamble and Telemark terranes, Sveconorwegian orogenic belt, SE Norway, and the Grenville-Sveconorwegian loop

The ~ 1100 Ma Sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against, and onto, the south-western margin of the Fennoscandian Shield. To resolve the magnetic signature acquired during post-orogenic uplift and cooling and evaluate wider correlations with the contemporaneous Grenville belt of North America, this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium-high metamorphic grade Bamble terrane (48 sites and 390 cores) and the adjoining medium-low grade Telemark terrane (33 and 240 cores) juxtaposed by an orogen-parallel (Porsgrunn- Kristiansand) fault zone with major vertical displacement. Magnetite and ilmeno-hematites are magnetic carriers with the latter more significant in the higher metamorphic grades. Magnetic intensities are stronger in the higher-grade terrane presumably due to the growth of metamorphic ferromagnets, but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust. Anisotropy of magnetic susceptibility (AMS) largely reflects the NE–SW tectonic grain of the last stage of Sveconorwegian ductile deformation. The magnetisation record is filtered by excluding magnetisations possibly acquired during regional Mesozoic dyke emplacement, development of the Permo-Carboniferous Oslo Rift and Late Proterozoic magmatism. The remaining record is a dual polarity signature summarised by mean poles at 31.9°N, 50.9°E, ( N = 191 components) in the Bamble terrane and at 34.2°N, 58.9°E ( N = 151 components) in the Telemark terrane. However these means are non-Fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best-defined parts comprising clockwise trajectories correlating with each another but indicating that cooling in Telemark was more protracted; in each case directions of more shallow NW-direction tend to be derived from lower unblocking temperature components. The geochronological evidence indicates that regional temperatures had fallen to permit acquisition of magnetisation by ~ 950–900 Ma and the two swathes define the younger limb of a clockwise (Grenville-Sveconorwegian) APW loop embracing the approximate interval 940–850 Ma; the outward path of this loop (~ 1020–940 Ma) is probably at present recorded only in dyke swarms from the Finnish sector of the shield. Correlation of APW between Laurentia and Fennoscandia confirms that the two shields broke apart shortly after culmination of the Sveconorwegian orogeny when Fennoscandia rotated rapidly clockwise into a secondary configuration adjacent to the eastern margin of Laurentia; the Grenville and Sveconorwegian orogenic frontal zones formed in alignment were reoriented at a high angle to one another in a coupling that appears to have persisted during most of the remainder of Neoproterozoic times.

Palaeomagnetic evidence and tectonic origin of clockwise rotations in the Yangtze fold belt, South China Block

Fold axis strikes in the Yangtze fold belt of the South China Block (SCB) undergo significant changes over distances of >1000 km. This large-scale variation provides an ideal opportunity to test the oroclinal-bending hypothesis using palaeomagnetic methods, which we have attempted by drilling the Lower Triassic Daye Formation limestones in western Hubei Province. Thermal demagnetization isolated two components in most samples. A low unblocking temperature component ( 400 °C , unblocks univectorially towards the origin. The HTC passes the McFadden-fold test with an overall mean tilt-corrected direction of Dec = 255°, Inc =-24° (N= 7, α95= 9°) . Scanning electron microscopy (SEM) observations suggest an early diagenetic, possible (bio)chemical origin for the magnetic extracts dominated by Ti-poor magnetite. Rock magnetic data show no evidence that the HTC has been affected by tectonic or compaction strain. Our data together with previously published results suggest a general clockwise rotation pattern in the Middle Yangtze fold belt, which is probably related to the collision between the North and SCBs. Comparison of palaeomagnetic rotations with fold axis trends in the fold belt suggests that about 30° clockwise rotation occurred in the Middle reaches of the Yangtze River, while a 15° difference in fold axis trends would be due to initial variation within the fold belt. However, since little is known about the timing of the clockwise rotation, whether the Middle Yangtze fold belt is an orocline awaits further studies.

Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.

Remagnetization of Mesozoic limestones from the Jaisalmer basin, NW India

SUMMARY High coercivity but low unblocking (LB) temperature components (∼80–100 °C) demonstrate that goethite is the principal carrier of the natural remanent magnetization (NRM) in Jurassic and Cretaceous limestone from the Jaisalmer basin (Rajasthan, NW India). Goethite-bearing components (declination = 359.9°, inclination = 46.0°, α95= 4.0°, N = 10 sites) plot close to the present day field direction. High unblocking (HB) components (declination = 172.0°, inclination =−45.6°, α95= 9.1°, N = 7 sites) are carried by haematite, mostly with reverse polarity directions, but undoubtedly not primary. This remagnetization event, mostly within a period of reverse magnetic polarity, occurred after 35 Ma and before 780 000 yr (the last known reversal). Only palaeomagnetic data from a single Cretaceous site (declination = 302.0°, inclination =−47.2°, α95= 14.0°, N = 10 samples) has potentially preserved a primary magnetization (carried by normal polarity magnetite); if correct, this site substantiates a mid-southerly palaeolatitude for India during mid-Aptian times (∼117 Ma).